1. Field of the Invention
The present invention relates to a developer sheet for forming high density images. More particularly, the developer sheet contains a developer material on its upper surface which, upon the application of pressure, does not significantly deform.
2. Description of the Prior Art
Transfer carbonless copy systems and transfer imaging systems are well known in the art. These systems comprise two sheets which are used to form a visible image. The first sheet, the transfer or donor sheet, typically contains on one of its surfaces, a colorless color-forming agent. Often, the color-forming agent is contained in pressure rupturable microcapsules. The second sheet, the developer or receiver sheet, typically is a substrate having a layer of a developer material which reacts with the colorless color-forming agent to form a visible image coated onto its front surface. In practice, to produce an image, the two sheets are aligned so that the colorless color-forming agent faces the developer material. Upon the application of pressure, the colorless color-forming agent reacts with the developer material on the developer sheet to form a visible image.
For a particular technical application, photosensitive imaging systems employing microencapsulated radiation sensitive compositions which are the subject of commonly assigned U.S. Pat. No. 4,399,209 to The Mead Corporation as well as copending U.S. patent application Ser. No. 320,643 filed Jan. 18, 1982, now U.S. Pat. No. 4,440,846, (corresponding to U.K. 2,113,860) have been developed. These imaging systems are characterized in that an imaging sheet including a layer of microcapsules containing a photosensitive composition in the internal phase is image-wise exposed to actinic radiation. In the most typical embodiments, the photosensitive composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated with a color former. The image-wise exposure hardens the internal phase of the microcapsules. Following exposure, the imaging sheet is assembled with a developer sheet and the assembly is subjected to a uniform rupturing force by passing the sheets through the nip between a pair of pressure rollers. Upon passing through the pressure rollers, the microcapsules image-wise rupture and release their internal phase whereupon the color former migrates to the developer sheet and forms an image. The imaging system can be designed to produce monochromatic or polychromatic full color images.
Although this system has proven to be quite successful, minor drawbacks still occur with respect to the developer sheet. More specifically, the developer material, which is typically a phenol formaldehyde novolak resin in particulate form, is susceptible to deformation as a result of the rupturing pressure. The deformation of the developer material on the receiver sheet is undesirable as this reduces the effective size of the pores between the particles of the developer material. According to a capillary model for oil transfer, the density of the image produced is proportional to the square root of the effective capillary radius of the developer material. In regions of high layer deformation, the effective capillary radius is smaller, and therefore, the density of the image is lower due to less efficient oil transfer in these areas.
In transfer systems, two types of materials are commonly used as developer sheets. The first material is a polymeric substrate, such as polyethylene terephthalate, coated with a developer material. The end product of such a receiver sheet is typically an overhead transparency. Because of the smooth and continuous surface of the polymeric substrate, when the developer material is coated onto the substrate, a uniform coating of particulate material is obtained. As such, when the developer sheet is assembled with the imaging sheet and subjected to pressure, the developer material uniformly deforms, thereby uniformiy decreasing the pore size between developer particles. Accordingly, it has been particularly difficult to obtain. high density images on clear polymeric substrates.
Alternatively, the other material traditionally used as a receiver sheet is paper. Depending on the paper selected, the texture of the surface of the paper containing the developer material often is discontinuous. For most photographic uses, the surface which contains the developer material has been pre-coated with a polymer coating to simulate the properties of the polymeric developer sheet. When the developer material is coated onto the polymeric coating, there are regions where the developer material is located at a greater paper thickness than in other regions. When the developer sheet is passed through pressure rollers, it has been discovered that the particles in the higher paper thickness regions are more likely to deform. Accordingly, in the lower paper thickness regions, a higher image density is achieved. Thus, the use of paper as a receiver material to some extent can produce a higher density image, and certainly a higher density image than that produced when using a polymeric receiver sheet. However, due to the substantial deviations in thickness of the paper, the images on the paper having high density versus the images having a low density are much more pronounced. Accordingly, the produced images can have a mottled appearance. This too is undesirable for obvious reasons.
A requirement of the above described transfer system is that a sufficient pressure must be exerted on the microcapsules to enable them to rupture and release their contents. As a result of the high rupturing pressure, it is inevitable that some developer material deformation will occur. One could hypothesize that to improve the density of the produced image the rupturing pressure should be lowered. This does not necessarily result. If too low a pressure is used, not all of the image-forming microcapsules will rupture, and as a result, the desired high density images will not be obtained. In addition, not every point of the donor or imaging sheet contacts every point of the developer sheet. As a result, high pressures must be maintained across the donor or imaging sheet and developer sheet to insure a uniform contact. Accordingly, other means must be provided for remedying the pressure deformation defect of the developer sheet.
Thus, there exists a need in the art for developer sheets which are capable of producing high density images and wherein the images produced are not mottled.